1. Field of the Invention
The present invention relates to a projecting optical system such as used in an image projecting apparatus to project an image formed on a primary image plane onto a secondary image plane.
2. Description of the Prior Art
Image projecting apparatuses employ a projecting optical system to project an image formed on a primary image plane, such as a liquid crystal display, onto a secondary image plane, such as a screen.
A projecting optical system which obliquely projects the image formed on the primary image plane onto the secondary image plane is well known.
Backward-projection-type projectors, for example, Head-Mounted Displays (HMDs), employ such obliquely projecting optical systems. HMDs and Heads Up Displays (HUDs) employ projecting optical systems that use a decentered mirror to secure a wide angle of view. In such projecting optical systems, a decentered combination of centered optical systems is used to cancel out asymmetric distortion (so-called trapezoid distortion) due to the inclination of the secondary image plane or caused by the decentered mirror. In other words, trapezoid distortion occurring in such projecting optical systems is corrected by the use of an optical system that is not symmetrical about its optical axis, that is, an optical system consisting of a plurality of centered optical systems that are combined such that their optical axes (axes of symmetry) do not coincide with one another.
When an optical system that is not symmetrical about its optical axis as described above is provided with a magnification varying function (for example, a zoom function) for varying the image magnification without changing the position of the secondary image plane (for example, in order to constantly obtain a sharp image on a plane on which the image is projected), it is essential that trapezoid distortion not appear even when the image magnification is varied.
For example, Japanese Laid-open Patent Application No. H5-119395 proposes a projection display apparatus that can vary the image magnification without causing trapezoid distortion. This apparatus is provided with a first projecting optical system that forms an intermediate image including trapezoid distortion, and a second projecting optical system that converts the intermediate image into an image free from trapezoid distortion. In this apparatus, the image magnification is varied by moving the second projecting optical system along the principal ray direction, and, at the same time, both of the first and second projecting optical systems are rotated to correct trapezoid distortion without changing the position of the secondary image plane.
On the other hand, Japanese Laid-open Patent Application No. H6-324285 proposes a visual display apparatus that varies the image magnification with a simple mechanism. This apparatus is provided with a decentered optical system, which includes a magnification varying lens unit so arranged as to be movable along its optical axis. In this apparatus, the image magnification is switched by moving the magnification varying lens unit along its optical axis such that its magnification becomes .beta. or 1/.beta..
However, such complicated movement of optical elements as required in the apparatus proposed in Japanese Laid-open Patent Application No. H5-119395 can be achieved only by the use of a complicated mechanism, and thus it is impossible to realize a compact and inexpensive projecting optical system according to this proposal. On the other hand, in the apparatus proposed in Japanese Laid-open Patent Application No. H6-324285, the switching of the image magnification is possible only in two steps, and thus it is impossible to obtain continuously varying image magnifications in a projecting optical system according to this proposal.